The major goal of this R01 proposal is to understand how heparan sulfate proteoglycans (HSPGs) regulate the formation of chemokine gradients, recruitment of inflammatory cells, and bacterial killing mechanisms in bacterial pneumonia. Leukocyte trafficking is a highly selective, nonrandom process and chemokines play a major role in guiding the directional migration of leukocytes from the bloodstream to the site of inflammation. Chemokine gradients are thought to form because chemokines move away from the source of production by diffusion and get tethered to cell surface and matrix HSPGs on the way. While quite a bit is known about the heparan sulfate (HS) binding epitopes on chemokines, less is known about the structural features of HS that enable HSPGs to bind and regulate chemokines. Moreover, HSPGs are biologically active molecules, expressed in distinct spatiotemporal patterns in tissues, and HSPGs themselves are highly regulated during an inflammatory response, but how these mechanisms impact HSPG-chemokine interactions in vivo are largely unknown. Furthermore, recent studies suggest that certain bacteria subvert HSPGs to inhibit host defense. Preliminary studies showed that early neutrophil recruitment to the lung is increased in syndecan-1 null (Sdc1-/-) mice infected with Staphylococcus aureus compared to wild type (Wt) mice, which led to increased clearance and significantly decreased bacterial burden in Sdc1-/- lungs. The basis of these gain of function phenotypes was traced to an increased binding of chemokines by cell surface HSPGs of type II epithelial (AT2) cells and increased steepness of the immobilized chemokine gradient from the endothelium to the alveolar epithelium in infected Sdc1-/- lungs. Based on these data, we hypothesize that: i) certain structural features of HSPGs expressed in different lung compartments differentially regulate the formation and activity of chemokine gradients; ii) syndecan-1 inhibits the formation of immobilized haptotactic chemokine gradients; and that iii) S. aureus subverts syndecan-1 to inhibit the formation and activity of chemokine gradients and to inhibit innate immune defense mechanisms. These hypotheses will be tested in 3 specific aims. Aim 1 will determine the structural features of how HSPGs regulate chemokines in bacterial pneumonia. Aim 2 will define how syndecan-1 inhibits chemokine gradients and neutrophil recruitment in bacterial pneumonia. Aim 3 will determine the underlying mechanisms of how syndecan-1 inhibits neutrophil-mediated defense mechanisms in bacterial pneumonia. These studies are expected to significantly refine the current chemokine gradient concept and to reveal new mechanisms of how bacteria subvert HSPGs for their pathogenesis in the lung.